The two leading unionist parties have formed their first major electoral pact in almost 30 years.

The DUP and UUP have struck a historic deal which will mean a single unity candidate standing in four constituencies across Northern Ireland.

However, the far-reaching agreement does not include South Belfast, regarded as one of unionism's best chances of snatching a seat from nationalists.

Following six months of secretive talks, the agreed candidates are Gavin Robinson in East Belfast, Tom Elliott in Fermanagh and South Tyrone, sitting MP Nigel Dodds in North Belfast, and Danny Kennedy in Newry and Armagh.

News of the pact immediately prompted Sinn Fein to call for the SDLP to consider a nationalist deal. Outgoing MP Conor Murphy said the SDLP must put progressive politics before its own narrow party political interests.

"This demands a strategic response from those of us who wish to see a society based on equality, inclusion and the protection of the most vulnerable," he declared. "Martin McGuinness will be speaking to the SDLP leader Alasdair McDonnell at the earliest opportunity to explore an appropriate and strategic response to this pro-Tory alliance."

One seat likely to be discussed is Mr McDonnell's own South Belfast seat, where the entry of Sinn Fein's Mairtin O Muilleoir has the potential to substantially reduce his vote. However, unionists were unable to agree a joint candidate, which would make a nationalist deal more imperative.

Nor was there any unionist pact in Upper Bann, where DUP man David Simpson is the sitting MP. He faces a tough battle with Sinn Fein to keep his seat.

DUP leader Peter Robinson and UUP chief Mike Nesbitt made the announcement in a joint statement, adding that agreement in South Belfast and Upper Bann "had not been possible".

Mr Robinson said all those who support the Union "will recognise the significance of this pact".

He termed it as the "most comprehensive electoral agreement between our two parties in the last 29 years".

"I want to see unionists co-operating," he said. "Too often divisions are manufactured to create difference. Grassroots unionists want to see us working together to maximise the unionist vote.

"Such an approach is not just desirable, but it has been proven to be effective in increasing turnout amongst unionists."

Mr Robinson commended the individual constituency associations from both parties whom he said "considered the bigger picture and accepted they should stand aside in the greater interests of unionism".

The First Minister said the agreement was particularly significant in light of the predicted hung parliament, where none of the main UK parties is expected to attract a big enough majority to form a government without the support of coalition partners.

"I am calling on all unionists to unite behind these agreed candidates and maximise the pro-Union voice in the House of Commons," he said.

"Sinn Fein MPs fail to take their seats. They leave their constituents without a voice in Parliament. Others do not represent unionism.

"All shades of unionism can lend their vote to these agreed candidates in the knowledge that their action could increase the number of unionists in the House of Commons and reduce the number of non-unionists returned."

Mr Nesbitt revealed the pact discussions began last October over Fermanagh and South Tyrone and North Belfast, before expanding to include other constituencies. He claimed there is "potentially a once in a lifetime opportunity" for a unionist to win Newry and Armagh, although he conceded it would be difficult.

"Our support for the DUP in East Belfast should ensure an additional pro-Union MP for the city of Belfast in the next mandate," he said.

Last night, Alliance MP Naomi Long accused the DUP of desperation and said the UUP had "abandoned their voters" in East Belfast. "The DUP clearly now know that Gavin Robinson cannot win against Alliance in a fair fight, so they have resorted to this anti-democratic move in a desperate attempt to strengthen their position," she said.

"We have seen unionist pacts before - this did not work in Fermanagh-South Tyrone in 2010. It will be the same in East Belfast in 2015."